To start internal combustion engines, they are accelerated to a minimum starting rotational speed, and are subsequently started using an injection of fuel and ignition of the fuel. In the usual vehicles, whose sole drive assembly is an internal combustion engine, the internal combustion engine is started at the beginning of the trip and is normally shut down again after the vehicle destination has been reached. European Patent No. EP 1 173 674 describes that the initial torque required for overcoming the starting resistances and for accelerating the power train masses of the internal combustion engine is of different magnitudes. The magnitude of the starting resistances, such as the internal combustion engine compression, the piston friction and the bearing friction are furthermore functions, for instance, of the number of cylinders and the model of the internal combustion engine, the lubricant used, the current temperature and the starting motion. Thus, at extremely low temperatures, a particularly large initial torque has to be overcome, whereas an engine that is at operating temperature is able to be started at a lesser torque.
Starting systems for internal combustion engines are therefore dimensioned so that a reliable start is possible even at unfavorable conditions. At each start of the internal combustion engine, the maximum available energy is therefore made available, in order to accelerate the internal combustion engine to a minimum starting rotational speed, using its maximum torque, and thus to make possible a start, as described above, to the internal combustion engine.
In the case of hybrid vehicles, the start of the internal combustion engine may take place not only at the beginning of the trip, but at any time, at the latest, however, when driving purely electrically is no longer possible. The reason for this may be a battery charging state that is too low and/or a command by the driver that can no longer be implemented purely electrically. This may lead to very frequent starting and stopping procedures of the internal combustion engine during a vehicle trip.
In order for a reliable start of the internal combustion engine to be possible, a motor, particularly an electric machine, has to be present in the hybrid vehicle, which is able to raise the required starting torque at any time. If the internal combustion engine of a hybrid vehicle is started during the electrical driving operation, the start of the internal combustion engine impairs the electrical driving operation in such a way that a part of the available electrical energy is used for the start of the internal combustion engine. As a result, in the purely electrical driving operation, either this electrical energy has to be withdrawn from the vehicle's driven end of shaft power and/or it is constantly set aside or held in reserve for the internal combustion engine's start. This restricts the electrical driving operation, as a rule, for reasons of comfort, since the energy set aside or held in reserve may not, or rather cannot be used for electrical driving. Thus, for operating the vehicle, particularly in a torque-based control device, it is advantageous if one knows precisely the currently required starting torque at a current at a current rotational speed, in order to make possible a reliable internal combustion engine start and a maximally extended electrical driving operation. Multiplying the internal combustion engine starting torque by the engine's rotational frequency gives the engine's starting power. The engine's rotational frequency may be determined from the engine speed by using the circle constant n.